The present invention relates generally to a variable reluctance magnetic transducer and, in particular, to a passive-type magnetic transducer especially suited for sensing the speed of a rotating object.
Passive-type variable reluctance magnetic transducers are well known and have been widely used as speed sensors in electric control circuits for monitoring the speed of an associated rotating object such as, for example, a gear. Typically, the transducer includes an outer housing which contains a permanent magnet, a pole piece constructed of a ferrous material and having one end positioned adjacent the magnet, and a coil surrounding the pole piece for sensing changes in magnetic flux through the coil. The opposite end of the pole extends outwardly from the housing and is adapted to be spaced from a projecting ferrous element which is attached to and rotates with the rotating object. For example, the projecting ferrous element or elements can be the teeth of a rotating gear. The coil is connected to generate an output signal to an associated electronic control circuit. An example of a prior art passive-type magnetic speed sensor can be found in U.S. Pat. No. 3,890,517.
In operation, a magnetic field extends from the magnet through the pole piece and out into the air space at the end of the pole piece. The return path of the magnetic field is from the air space to the other end of the magnet. As a ferrous element approaches the tip of the pole piece, the reluctance of the magnetic circuit decreases, thus increasing the magnetic field. As the ferrous object passes away from the pole piece, the magnetic field decreases. When the magnetic field decreases, it induces a voltage in the coil in one direction and, when it increases, it induces a voltage in the coil in the opposite direction. The passage of one ferrous object (such as one gear tooth) induces one cycle of AC voltage. The AC voltage is proportional to the rate of change of magnetic flux in the magnetic circuit, and is generally proportional to the speed of the ferrous objects passing the pole piece, at least up to a predetermined speed. The frequency of the AC signal is directly proportional to the number of ferrous objects passing the pole piece per unit of time.
The output voltage of the speed sensor is typically an important design criteria, i.e., the speed sensor must generate a sufficient output voltage at the lowest speed of interest. The output voltage of the speed sensor depends on a wide variety of factors, some of which include the surface speed of the gear teeth passing the pole piece, the gap between the pole piece and the gear teeth, the size and shape of the gear teeth, the spacing between the gear teeth, the load impedance connected to the speed sensor, and the particular configuration of the pole piece.
Generally, in order to obtain the highest output from the speed sensor, the pole piece should be located as close to the gear as the particular application will permit. The output voltage of the speed sensor is significantly affected by changes in the air gap and is almost inversely proportional to the square of the air gap.
Changes in the configuration of the pole piece can also affect the speed sensor output voltage. Generally, when sensing a rotating gear, sensors are provided with a cylindrical projecting portion having a diameter which is less than or equal to the width of the top surface of the associated gear tooth. However, when using a gear of finer than optimum pitch, it has been found desirable to use a pole piece having an outer tip shaped to a chisel, and which is oriented perpendicular with the rotational axis of the gear.